Lithographic printing conventionally involves the use of a printing press having one or more printing cylinders. A lithographic plate, which is typically of rectangular configuration and made from thin metal, is used for each printing cylinder. The lithographic plates are replaceably mounted on the printing cylinder and a separate plate must be prepared for each printing cylinder for each job (usually), each lithographic plate represents one page of text and/or illustrations.
As is well known in the art, preparation of a lithographic plate involves coding it with a photosensitive material, followed by exposure to light through an image (e.g., film) and development of the plate. As a result, the developed positive image attracts ink, while ink is repelled by the negative portion (the converse may also occur depending on how the image is formed).
The developed lithographic plate is then mounted on the cylinder in encircling relation and clamped tightly to its surface, and the printing process begins.
A problem that has long existed in the production of lithographic plates is properly orienting the image relative to the lithographic plate prior to the exposure to light. Typically, the image must be centered on the center of the printing area of the lithographic plate. Conventionally, this has been accomplished by determining the printing area of the particular plate being used, and superimposing an image carrier over the plate with the image properly positioned relative to such printing area.
A lithographic plate must be sized and configured relative to the printing cylinder with which it is used. The size is determined by the printing cylinder circumference and axial length. The configuration involves bending the leading and trailing edges of the plate to fit into clamping apparatus within the printing cylinder. The printing area is disposed between the bends formed at each end of the plate. Since the bends are generally of different configuration at each end, the printing area is not centered within the plate; i.e., the plate center and printing area centers do not coincide.
This obviously must be taken into consideration in positioning the image. The printing area center is established by measuring, and the image is then appropriately positioned relative to the printing area center. This is usually done on a working surface by measuring and then taping the image carrier in place over the lithographic plate.
Each image must be individually positioned over the associated lithographic plate, and this procedure requires considerable time. The problem is compounded in large printing operations involving multiple presses with a substantial number of printing cylinders, which often are of different size and configuration. This involves the use of lithographic plates of different size and bend configurations, and unique measuring and image placement is required for each. Consequently, the production of lithographic plates with known procedures is both time consuming and costly.